Trona is a naturally occurring sodium sesquicarbonate (Na2CO3.NaHCO3O.2H2O). The Green River basin in southwestern Wyoming contains the world's largest known deposit of trona. Reserves in Wyoming amount to approximately 140 billion tons. In the Green River Basin there are approximately twenty-five beds of trona more than four feet thick with intervening strata of shale. These beds are encountered at a below surface depth between 500 and 3000 feet.
Globally, soda ash (i.e., sodium carbonate, Na2CO3 or SC) is a 54 million metric tons per year commodity. Synthetic soda ash manufactured from limestone and salt accounts for 73% of the global production. The remaining 27% is generally referred to a natural soda ash as it is produced from naturally occurring deposits of trona.
Trona is the principle source mineral for the United States soda ash industry and is generally produced by conventional underground mining methods, including solution mining. Non-solution mined ore is hoisted to the surface and is commonly processed into soda ash either by the “sesquicarbonate process” or the “monohydrate process.” In the sesquicarbonate process, the processing sequence involves underground mining; crushing; dissolving raw ore in mother liquor; clarifying; filtering; recrystallizing sodium sesquicarbonate by evaporative cooling; and converting to a medium density soda ash product by calcining. The monohydrate process involves underground mining, crushing; calcining of raw trona ore to remove carbon dioxide and some organics to yield crude soda ash; dissolving the crude soda ash; clarifying the resultant brine; filtering the hot solution; removing additional organics; evaporating the solution to crystallize sodium carbonate monohydrate; and drying and dehydrating sodium carbonate monohydrate to yield the anhydrous soda ash product.
Solution mining of trona, such as taught by Day (U.S. Pat. No. 7,611,208) minimizes the environmental impact and reduces or eliminates the cost of underground mining, hoisting, crushing, calcining, dissolving, clarification, solid/liquid/vapor waste handling and environmental compliance.
Trona and nahcolite are the principle source minerals for the United States sodium bicarbonate (“SBC”) industry. Sodium bicarbonate is produced by nahcolite solution mining or water dissolution and carbonation of mechanically or solution-mined trona ore or the soda ash produced from that ore. As taught by Day (U.S. Pat. Nos. 4,815,790 and 6,660,049), sodium bicarbonate is also produced by solution mining nahcolite, the naturally occurring form of sodium bicarbonate. Nahcolite solution mining utilizes directionally drilled boreholes and a hot aqueous solution comprised of dissolved soda ash, sodium bicarbonate and salt. In either case, the sodium bicarbonate is produced by cooling or a combination of cooling, and carbonation crystallization.
Kube in U.S. Pat. No. 3,953,073 teaches solution mining trona using sodium hydroxide to prevent “severe solubility suppression resulting, at least in part, from clogging of the dissolving face by sodium bicarbonate.” Kube provides a 30° C. example (column 5, lines 14-39) of the benefit of using sodium hydroxide to prevent the solvent from contacting a “virtually impenetrable barrier of sodium bicarbonate.” The solution in contact with the SBC barrier is saturated at 6.7% SBC and 8.4% SC (12.6% total alkalinity reported as SC (TA)) whereas a saturated solution in contact with the non-encapsulated trona is saturated at 4.6% SBC and 17.3% SC (20.2% TA). SBC encapsulation can reduce trona solution mining productivity by about 40% (20.2% to 12.6%). Kube estimates the quantity of the SBC encapsulating the trona is 12.4 grams per 100 grams of water. Kube teaches the use of sufficient sodium hydroxide to convert 12.4 grams of SBC to SC to eliminate the encapsulating SBC and the solubility suppression. There are no commercial applications of Kube's invention. Commercial trona solution mining operations simply accept the solubility suppression described by Kube.
Therefore, there remains a need in the art for improved methods of solution mining for trona, to allow for recovery of a solution that is rich in desired dissolved minerals and lean in undesired dissolved minerals leading to more cost-effective commercial products from the solution, improved resource recovery, and reduced environmental impacts compared to conventional underground mining.
A problem with the use of the current aqueous trona solution mining methods is clogging of the trona dissolution surface caused by dissolved SBC supersaturation, and nahcolite and/or wegscheiderite precipitation, as the solution approaches double saturation. Double saturation refers to the condition where both dissolved SBC and SC are at saturation. The supersaturation and precipitation occur because, as trona dissolves in water or in the mixtures of SC, SBC, and water of the current solution mining practices, the solution becomes supersaturated in respect to dissolved SBC as the solution approaches double saturation. This is commonly referred to as incongruent dissolution. Supersaturated dissolved SBC can precipitate as either nahcolite or wegscheiderite on the face of the trona, clogging the trona dissolution surface and practically stopping the dissolution of trona. This hinders the resource recovery, the solution mining process, and the economics. Thus, there is a need for economical methods to eliminate or manage the consequences of the clogging.